igc: avoid returning frame twice in XDP_REDIRECT

In the Linux kernel, the following vulnerability has been resolved:

igc: avoid returning frame twice in XDP_REDIRECT

When a frame can not be transmitted in XDP_REDIRECT
(e.g. due to a full queue), it is necessary to free
it by calling xdp_return_frame_rx_napi.

However, this is the responsibility of the caller of
the ndo_xdp_xmit (see for example bq_xmit_all in
kernel/bpf/devmap.c) and thus calling it inside
igc_xdp_xmit (which is the ndo_xdp_xmit of the igc
driver) as well will lead to memory corruption.

In fact, bq_xmit_all expects that it can return all
frames after the last successfully transmitted one.
Therefore, break for the first not transmitted frame,
but do not call xdp_return_frame_rx_napi in igc_xdp_xmit.
This is equally implemented in other Intel drivers
such as the igb.

There are two alternatives to this that were rejected:
1. Return num_frames as all the frames would have been
transmitted and release them inside igc_xdp_xmit.
While it might work technically, it is not what
the return value is meant to represent (i.e. the
number of SUCCESSFULLY transmitted packets).
2. Rework kernel/bpf/devmap.c and all drivers to
support non-consecutively dropped packets.
Besides being complex, it likely has a negative
performance impact without a significant gain
since it is anyway unlikely that the next frame
can be transmitted if the previous one was dropped.

The memory corruption can be reproduced with
the following script which leads to a kernel panic
after a few seconds. It basically generates more
traffic than a i225 NIC can transmit and pushes it
via XDP_REDIRECT from a virtual interface to the
physical interface where frames get dropped.

#!/bin/bash
INTERFACE=enp4s0
INTERFACE_IDX=`cat /sys/class/net/$INTERFACE/ifindex`

sudo ip link add dev veth1 type veth peer name veth2
sudo ip link set up $INTERFACE
sudo ip link set up veth1
sudo ip link set up veth2

cat << EOF > redirect.bpf.c

SEC("prog")
int redirect(struct xdp_md *ctx)
{
return bpf_redirect($INTERFACE_IDX, 0);
}

char _license[] SEC("license") = "GPL";
EOF
clang -O2 -g -Wall -target bpf -c redirect.bpf.c -o redirect.bpf.o
sudo ip link set veth2 xdp obj redirect.bpf.o

cat << EOF > pass.bpf.c

SEC("prog")
int pass(struct xdp_md *ctx)
{
return XDP_PASS;
}

char _license[] SEC("license") = "GPL";
EOF
clang -O2 -g -Wall -target bpf -c pass.bpf.c -o pass.bpf.o
sudo ip link set $INTERFACE xdp obj pass.bpf.o

cat << EOF > trafgen.cfg

{
/* Ethernet Header */
0xe8, 0x6a, 0x64, 0x41, 0xbf, 0x46,
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
const16(ETH_P_IP),

/* IPv4 Header */
0b01000101, 0, # IPv4 version, IHL, TOS
const16(1028), # IPv4 total length (UDP length + 20 bytes (IP header))
const16(2), # IPv4 ident
0b01000000, 0, # IPv4 flags, fragmentation off
64, # IPv4 TTL
17, # Protocol UDP
csumip(14, 33), # IPv4 checksum

/* UDP Header */
10, 0, 1, 1, # IP Src - adapt as needed
10, 0, 1, 2, # IP Dest - adapt as needed
const16(6666), # UDP Src Port
const16(6666), # UDP Dest Port
const16(1008), # UDP length (UDP header 8 bytes + payload length)
csumudp(14, 34), # UDP checksum

/* Payload */
fill('W', 1000),
}
EOF

sudo trafgen -i trafgen.cfg -b3000MB -o veth1 --cpp